On a recent hike along the Signal Hill trail system I noticed, other than the amazing views of the entrance to the St. John's Harbour aka "The Narrows" was the prominent Quartz veins in this area.
The first thing that cam,e to mind is how I could place a new Earthcache here along the trail system! No need to even leave the trail to spot this one either!! Added Bonus...lol
What is a Quartz Vein?
Quartz is a defining constituent of granite and other felsic igneous rocks, and this is what you see here today. As individual crystals it is very common in sedimentary rocks such as sandstone and shale and is also present in variable amounts as an accessory mineral in most carbonate rocks. It is a common constituent of schist, gneiss, quartzite and other metamorphic rocks. Quartz has the lowest potential for weathering, consequently it is very common as a residual mineral in stream sediments and residual soils. There are various ways quartz can crystallise, but the majority of quartz crystallises from molten magma. Well-formed crystals may reach several metres in length and weigh hundreds of kilograms, unfortunately, you won’t see one that big here as we are only viewing quartz veins.
As well as crystals, Quartz can also occur in what appear to be veins, and this is how they are usually described. The term is slightly misleading, as the ‘veins’ you see are not thin tubes, they are usually thin sheets but because we only see the cross section they appear to be veins. The veins can form under various conditions, and depending on these conditions, may or may not bear quartz crystals in them.
The simplest type of a quartz vein is the filling of an already present crack in rocks. The crack might form for a number of different reasons, which include:
- Cracks formed during folding of the rock in a mountain-building process
- Cracks formed by shattering during tectonic events (movements in the earth’s crust)
- Cracks formed by a decrease in pressure during the uplift of a rock
- Cracks formed when a hot rock cools down and shrinks.
At some point, the rocks will have been greatly heated due to changes in the earth. In this area, volcanic activity is quite possibly involved. Hot brines will filter gradually through the already cracked rocks to greater depths. The process will filter the minerals the brines carry into the cracks where they will remain as the temperatures and pressure drop. This process may continue until the crack is completely filled or may stop before, leaving "pockets" in the vein that are sometimes outlined by crystals.
So why is there a variation in the colour of quartz veins? Hot brines that enter a crack in the rock from some distant hot source often cool and filter most of their load rather quickly. When that happens, the result is milky coloured quartz. Milk quartz or milky quartz is the most common variety of crystalline quartz. The white colour is caused by minute fluid inclusions of gas, liquid, or both, trapped during crystal formation and this makes it of little value for optical and quality gemstone applications.
Later, when the crystal growth slows down, the crystals may get less milky or even clear. In the majority of quartz veins, most of the quartz is formed as massive, milky quartz, and well-formed crystals, will be at most only a small portion of the vein filling, and may well not be found at all. In some places here you will see some red colouring within the quartz. Like with the slate, this may be caused by a presence of iron.
Logging Requirements:
1. Take a look at the quartz veins that run along the rock face just above head level. What direction do they travel (N, S, E, or W)?
2. What is the approximate width of the quartz vein that is approx 10 feet up the rockface at GZ? You would be standing below the vein at this point.
3. Turn your attention to the colour. Describe the predominant colour you see in the veins. What does this tell you about how quickly the brine cooled?
4. Post a picture of youself or your GPS at the location of this Earthcache.